JPH0640759B2 - Restraint monitoring device when starting AC motor - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to an AC motor that is asynchronously started, and more particularly to a restraint monitoring device when the AC motor is started.

[Technical background of the invention and its problems]

The thermal withstand capability of an AC motor at asynchronous start varies greatly depending on whether the AC motor is constrained. This is because if the AC motor is restrained, the cooling effect due to the rotation of the rotor cannot be obtained. By the way, as a means for detecting the rotation of the rotor, a mechanical rotation detector has been conventionally considered, but if a rotation detector of this kind is provided, the price will increase considerably.

On the other hand, when such a rotation detector is not provided, the constraint protection at the time of starting the AC motor is determined by whether the starting current flows for the allowable constraint time or longer. Etc. cannot be protected. On the contrary,
When designing an AC motor, the AC motor must be designed in consideration of thermal resistance so that there is no problem even if the AC motor is restrained for more than the starting time, so the AC motor becomes larger than necessary. There is also a problem.

[Object of the Invention]

The present invention has been made to solve the above problems, and an object thereof is to provide a mechanical detection means for determining whether or not the AC motor is restrained when the AC motor is asynchronously started. Realized by monitoring only the current without
It is an object of the present invention to provide an inexpensive restraint monitoring device at the time of starting the AC motor, which can surely perform ideal restraint protection of the AC motor.

[Outline of Invention]

The present invention, in an AC motor that is asynchronously started, a current detector that detects the starting current of the AC motor, a phase detector that detects the phase of the starting current detected by this current detector, and this starting current. It has an integrator for integrating and an A / D converter for A / D converting the output of this integrator, and further reads and compares an integral value for every fixed period from the output of the phase detector and the output of the A / D converter. Accordingly, the constraint monitoring device at the time of starting the AC motor is characterized by having a constraint determiner for determining whether or not the AC motor is constrained.

Example of Invention

 First, the concept of the present invention will be described.

Generally, when an AC motor is asynchronously started, a DC current is superimposed on the stator side for a short time in addition to an AC current corresponding to the power supply frequency as a starting current. When the rotor of the AC motor starts to rotate, while the DC component current flows, the rotor is linked with the DC magnetic field due to the DC current of the stator, so that the voltage of the frequency corresponding to the rotation speed of the rotor itself is generated. Current is generated on the rotor side. Then, on the contrary, since the same number of voltages as this frequency is induced on the stator side,
The stator current is the starting current in which the low-frequency alternating current component is superimposed, and the current immediately after starting is not simply attenuated but undulates. The low-frequency AC component does not flow until the rotor starts rotating, so when the rotor is restrained, the current immediately after starting is simply attenuated. The phenomenon of whether or not the current immediately after the start is simply attenuated is determined by comparing the absolute values of the integrals of the current waveform for each constant cycle (for example, one cycle), whereby the rotor of the AC motor is restrained. It is possible to determine whether or not there is.

An example of the waveform of the stator current when the rotor is restrained is shown in FIG. 4, and an example of the waveform of the stator current when the rotor is not restrained is shown in FIG. In the figure, A shows the starting current, B shows the envelope thereof, and Q shows the electric starting point.

An embodiment of the present invention based on the above concept will be described below with reference to the drawings. FIG. 1 is a block diagram showing an outline of a configuration example of a restraint monitoring device at the time of starting the AC electric motor according to the present invention. The current transformer 3 for detecting the starting current of the AC electric motor 1 and the current transformer 3 detect the current. A phase detector 6 for detecting the phase of the starting current, an integrator 5 for integrating the starting current, an A / D converter 9 for A / D converting the integrator output,
The AC electric motor 1
2 shows a constraint monitoring device at the time of starting the AC motor, which is provided with a constraint determination unit 10 for determining the presence or absence of the constraint.

In FIG. 3, the AC motor 1 is connected via a breaker 2 to a bus bar leading to a power source (not shown). Further, the main circuit of the AC motor 1 is provided with a current transformer 3 as a current detector that detects a starting current. An I / V converter 4 is provided on the secondary side of the current transformer 3 for converting a current into a voltage level. The output of the I / V converter 4 integrates the input voltage and is supplied by an external input. The signal is output to the integrator 5 that periodically resets (discharges) and the phase detector 6 that detects the phase. The sign inversion unit 7 is a unit that inverts the sign of the output of the integrator 5, and the OR circuit unit 8 outputs the output of the integrator 5 and the output of the sign inversion unit 7 (the opposite sign to the output of the integrator 5). However, the absolute value is the same), the absolute value of the output of the integrator 5 is output by taking the logical sum. The absolute integrated value obtained by the logical sum circuit unit 8 is A / D
Through the A / D converter 9 that performs the conversion, the calculation is performed and the result is input to the constraint determination unit 10 that determines the presence or absence of the constraint.
The constraint determination unit 10 uses the phase signal input via the buffer unit 11 for level conversion of the output of the phase detection unit 6 to read the absolute integral value, for example, every cycle, and for level conversion. The integrator 5 is forcibly discharged through the buffer section 12 and the forcible discharge signal circuit section 13 of the integrating circuit to detect the absolute integral value of the waveform for each cycle and determine whether or not there is a constraint. .

Reference numeral 14 is an oscillating circuit that serves as a power source of the constraint discriminating unit 10, 15 is a buffer unit for inputting a closing command (a signal indicating that the circuit breaker 2 is turned on) to the constraint discriminating unit 10, and 16 is a constraint This is an output relay circuit section for outputting to the outside that the determination unit 10 has determined that the AC motor 1 is in the restrained state.

In the restraint monitoring device having such a configuration, when the breaker 2 is closed and the AC motor 1 is started to be asynchronously started, the secondary side of the current transformer 3 has a frequency component corresponding to the rotation speed of the rotor as described above. Since the current is also superposed on the initial starting current at the time of starting, the secondary current of the current transformer 3 is shown in FIG. 5 for a short time (for example, 0.3 to 0.5 sec) immediately after starting until the DC component of the main circuit is attenuated. Waviness occurs as shown in. Therefore, with respect to this secondary current, the integrator 5, the sign inversion unit 7, and the OR circuit unit 8
Then, the absolute integration value is read by the signal from the phase detection unit 6 into the constraint determination unit 10 for each cycle, for example, and the read absolute integration value is affected by the low-frequency AC component current. Increases or decreases without monotonically decreasing with.

On the other hand, when the AC motor 1 is restrained at the time of starting, the low-frequency AC component current is not induced, so the current transformer 3
On the secondary side of the, AC component (steady term) corresponding to the power supply frequency
The current and the current of the direct current component (attenuation term) flow only in a superposed manner, and the current waveform has a monotonically decreasing current as shown in FIG. For this reason, as described above, when the absolute integration value is read into the constraint determination unit 10 for each cycle, for example, as to the secondary current, the read absolute integration value monotonically decreases.

Therefore, the absolute value of integration is monitored by the constraint determination unit 10 for a certain period of time immediately after the start (for example, about 0.3 seconds when the DC component is attenuated to some extent), and whether or not the absolute integration value monotonously decreases is monitored. If determined, it can be determined whether or not the rotor is restrained.

Incidentally, shown At a Figure 3, for example, ICI operational amplifier in the integrator 5, _{C 1} denotes a condenser, the FET transistor Q1, the R2, R3 resistors.

Further, an example of a flow chart in the constraint discriminating unit 10 is shown in FIG. By configuring such a restraint monitoring device, it is possible to obtain an inexpensive restraint monitoring device by monitoring only the current.

The following effects can be obtained by using the restraint monitoring device configured as described above.

(a) Without providing a mechanical detector for the AC motor 1,
Detection of whether or not the AC motor 1 is restrained can be realized at low cost.

(b) Conventionally, when the starting time of the AC motor 1 is longer than the allowable restraint time of the motor, the restraint protection of the AC motor 1 cannot be performed. It can be distinguished from the start congestion protection only by monitoring the current, and even the electric motor having a long start time can be easily restrained and protected.

(c) In designing the AC motor 1, it is premised that distinction between restraint protection and start congestion protection is made, so that it is possible to design a simpler AC motor.

(d) Since the presence or absence of restraint can be detected in the initial stage of starting, it is possible to protect before the AC motor is subjected to excessive distortion as compared with the conventional protection device.

(e) Even when protecting an existing AC motor, it is possible to surely perform the protection protection of the AC motor without modifying the AC motor.

〔The invention's effect〕

As described above, according to the present invention, when the AC motor is asynchronously started, the determination as to whether or not the AC motor is restrained is realized only by monitoring the current without providing a mechanical detection unit. It is possible to provide an inexpensive and highly reliable restraint monitoring device at the time of starting the AC motor, which can surely perform ideal restraint protection.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram of a restraint monitoring device showing an embodiment of the present invention, FIG. 2 is a flow chart explanatory diagram showing an internal processing procedure of the restraint discriminator of FIG. 1, and FIG. 3 is another embodiment. The circuit configuration diagram shown in FIG. 4, and FIG. 4 and FIG. 5 are waveform explanatory diagrams showing the operation characteristics of FIG. 1 ... AC motor, 2 ... Breaker, 3 ... Current transformer, 4 ... I / V converter, 5 ... Integrator, 6 ... Phase detector, 9 ... A / D converter, 10 …… Restriction discrimination section, 16 …… Output circuit.

Claims (1)

[Claims] 1. In an AC motor that is asynchronously started, a current detector that detects the starting current of the AC motor, a phase detector that detects the phase of the starting current detected by this current detector, and this starter. An absolute integration value for every constant period from an integration unit that calculates the absolute integration value of the current, an A / D converter that A / D converts the output of this integration unit, and the output of the phase detection unit and the output of the A / D converter. And a constraint determination unit for determining the constraint state of the AC electric motor by reading and comparing the above.